| Membrane bioreactor (MBR) has drawed popular attention as a novel technology for the high-efficiency to treat the wastewater.However, membrane fouling significantly became an obstacle for the futher development and application of the MBR. The microorganism in MBR is in a relatively closed system, and the continued accumulation of microbial metabolites gradually becomes the major factor influencing effluent quality and membrane fouling,so research on membrane fouling mechanism and control methods of soluble microbial products (SMP) in sludge mixture has become an important content of applied research of MBR.Making the synthetic wastewater as the raw influent, the accumulation and characteristics of SMP during the start up of MBR were studied, thus give an interpretation to the formation mechanism of SMP in MBR from the microbial perspective. In addition, the influence of solid retention time (SRT) on the characteristics of SMP and the relationship between the microbial community and SMP were both studied. To investigate the effects of charactersitics of SMP on membrane fouling, Linear Regression analysis was conducted. Classic models of membrane flouing mechanism were applied to study the membrane filtration process in order to give an interpretation to the membrane fouling mechanism of SMP. Thus, the research could provide theory basis for MBR process optimization and membrane fouling control. The results were as follows:(1) The effects of microorganism growth phase (MGPs) on accumulation and characteristics of SMP in MBR were investigated. The results showed that there were three MGPs in the experiment, including exponential growth phase (EGP), deceleration growth phase (DGP) and stationary growth phase (SGP), respectively. Polysaccharides and proteins in SMP were steadily increased in the EGP. While in the DGP, the SMP were increased firstly and then decreased sharply. Finally, the concentrations of SMP maintained at a low steady level in the SGP. Furthermore, the biomass associated products (BAP) batch experiment showed that the production potential of SMP was the largest in the DGP. The percentage of SMP (>30kDa) was remarkably increased, then dropped due to abruptly decreasing production potential of BAP and degradation by acclimated microorganisms. The results analyzed by excitation-emission matrix fluorescence spectroscopy indicated that the variation of protein-like substances showed a similar trend with the total SMP, while the humic acid-like substances were related with microorganism slightly.(2) Accumulation of SMP in supernatant became more pronounced at short SRT. However, the variations of SMP in membrane fouling layer showed differences from the SMP in supernatant that the concentrations of SMP reached minimum at an intermediate SRT. Concentration of polysaccharide and protein had a significant variation in different SRT while the ratio between polysaccharide and protein was stable.The ratio between polysaccharide and protein in surpernatant was far below to the ratio in membrane fouling layer, which demonstrated that polysaccharide was prone to be attached to the membrane compared. Moreover, the percentage of high molecular weight and hydrophobic components in SMP increased with the prolonged SRT. The majority of SMP in membrane fouling layer was high molecular weitht and hydrophobic components. The protein-like fluorescence peak was the main peak in the excitation-emission matrix fluorescence spectroscopy of SMP in membrane fouling layer and the peak lacotion had a blue shift compared with the SMP in supernatant due to the degaration of microbial bacteria in membrane.(3) The hydrophobic fraction of SMP plays a more significant role than the hydrophilic fraction in membrane fouling. While compared with the size exclusion by high molecular weight fraction, the hydrophobic interaction by hydrophobic fraction was not the governing fouling mechanism.(4) Microbial community structure analysis and sequencing results show that the microbial community structure in membrane fouling layer was complex than that in sludge mixture under different SRT. The internal of membrane fouling layer can form anaerobic and anoxic mico-environment, thus the complex living environment in membrane fouling layer could be conducive to the growth of anaerobic bacteria.(5)The development of membrane fouling process was divided to two phases: one phase is membrane pore blocking and surface absorption and the other is the formation of cake layer. Moreover, the membrane resistance caused by cake layer was the main reason for membrane fouling and the effect of SMP on the membrane fouling was mostly contributed in the membrane resistance of cake layer. |
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